1. Field of the Invention
The present invention relates to an image scanning apparatus which comprises a plurality of monochrome line sensors and where color separation illumination is performed. More particularly, the present invention relates to an image scanning apparatus which is preferable for scanning an image in high quality with improvement in image scanning rate and in S/N of a color image scanning signal, and accurately determining white balance in each color and shading correction coefficient, and also relates to a storage medium which stores a controlling procedure of the image scanning apparatus, and a control program for the image scanning apparatus.
2. Description of the Related Art
There are two conventional image scanning apparatuses as follows.
An image scanning apparatus of the first type comprises a monochrome CCD line sensor, where a single line can be scanned at once.
Specifically, a luminous source switches in sequence the color of emitting light in the order of red (R), green (G) and blue (B), and emits lights of R-color, G-color and B-color in sequence onto an original document. Subsequently, the single CCD line sensor outputs in sequence an R-line image data which is scanned by using illumination light of the R-color, a G-line image data which is scanned by using illumination light of the G-color, and a B-line image data which is scanned by using illumination light of the B-color, thereby obtaining a color image scanning signal for one line in a main-scan direction.
Moreover, a sub-scan mechanism transfers a scanning position of the CCD line sensor to an adjacent line, where the R-line image data, G-line image data and B-line image data for the adjacent line are scanned as well. The above operations are repeated to obtain the color image scanning signal.
The second type of the image scanning apparatus uses 3-line color CCD line sensors, where three lines can be scanned at a time.
Specifically, by using the 3-line color CCD line sensors, a white luminous source is allowed to emit light once, whereby the R-line image data, G-line image data and B-line image data can be obtained simultaneously.
The image scanning apparatus using the monochrome CCD line sensor cannot simultaneously accumulate color components of the three colors (R-color, G-color and B-color) as charge in the CCD line sensor. Hence, it is necessary to emit light three times each for the R-color, G-color and B-color, in order to obtain the R-line image data, G-line image data, and B-color image data.
On the other hand, the image scanning apparatus with the 3-line color CCD line sensors can obtain the color image scanning signal by emitting white light only once.
Therefore, an image scanning rate of the image scanning apparatus using the single monochrome CCD line sensor is slower than that of the image scanning apparatus with the 3-line color CCD line sensors. On the other hand, the image scanning apparatus using the single monochrome CCD line sensor realizes an improvement in color reproduction by devising illumination spectral characteristic. However, the illumination area of the image scanning apparatus using the single monochrome CCD line sensor is extremely wide in a sub-scan direction, compared with its light receiving area. As a result, energy used for the illumination is not utilized efficiently, resulting in a waste of energy. Incidentally, it is possible to narrow the area where the light illuminates the original document, in the sub-scan direction. However, expensive components are needed for achieving that, which results in increasing a cost of the image scanning apparatus.
Moreover, when an image is scanned with the image scanning apparatus of the first type, that is, the illumination-switching-type image scanning apparatus using the single monochrome CCD line sensor, differences occur according to the transfer characteristics of the optical system. There is also a lack in uniformity in the sensitivity of the single monochrome CCD line sensor depending on the pixel positions.
Therefore, distortions occur both in distribution of the luminous energy of light which is incident from the original document and of the luminous energy of signals generated by the image scanning apparatus (R line image data, G line image data, B line image data). Unless correction is made to the distribution of the luminous energy of the generated signals by the image scanning apparatus, a phenomenon occurs due to the distortions that luminous energy is large on the central part of the image but small in the periphery of the image, compared to the original document. The conventional image scanning apparatus has shading correction mechanism in order to adjust the distortions in the luminous energy distribution.
Conventional shading correction mechanism will be explained in the following. An image scanning apparatus has, according to its characteristics, a memory for storing in advance a large number of shading correction coefficients which are determined in accordance with each luminescent color of the luminous source and each pixel on the surface of an imaging device (chip). At performing the imaging, a predetermined controlling device reads out a shading correction coefficient from the memory corresponding to a main-scan position, obtains shading-corrected image data by multiplying image data outputted from an imaging apparatus by the shading correction coefficient. Every time the main-scan position varies, the shading correction coefficient read from the memory varies accordingly.
Needless to say, the above shading correction is performed as well in the image scanning apparatus using the single monochrome CCD line sensor and monochrome light.
In the conventional image scanning apparatus using the single monochrome CCD line sensor, the shading correction coefficient is obtained for each pixel for each color, and when the monochrome light is used, for a single color.
However, when the shading correction coefficient is obtained in an image scanning apparatus using a plurality of monochrome CCD line sensors in the same manner as in the image scanning apparatus using a single monochrome CCD line sensor as described above, following problems arise.
That is, when image data in a single monochrome CCD line sensor included in a plurality of monochrome CCD line sensors is employed to obtain the shading correction coefficients by each pixel in the image scanning apparatus using a plurality of monochrome CCD line sensors, only the shading correction coefficient of the single monochrome CCD line sensor can be accurately obtained. However, it is not possible to obtain an accurate shading correction coefficient for the rest of the monochrome CCD line sensors in the plurality of monochrome CCD line sensors because image data of these rest of the line sensors are not taken into consideration.
Therefore, when the amount of light incident from an object into a plurality of monochrome CCD line sensors exceeds the saturation level of these rest of the monochrome CCD line sensors, overflowing charges to the adjacent pixel registers cause a phenomenon where an image is distorted (blooming). In addition, in a case where a white balance determined in a single monochrome CCD line sensor is used, when the levels of electric signals outputted from these rest of the monochrome CCD line sensors exceed the saturation level of a signal processing circuit such as an A/D converter, the image data with shading correction not yet made no longer vary and the relation between the level of the amount of input light and the image data is represented by a non-linear curve. This occurs due to different sensitivities of the CCD line sensors.
It is not possible to obtain an accurate correction result of performing the shading correction processing on the non-linear image data.
In recent years, the image scanning apparatus using a plurality of monochrome CCD line sensors has been in progress of development, and there has been a demand for techniques which realize accurate shading correction in this apparatus.